Note that these aren't their surnames - they're brothers, as Finn was adopted by Jake's parents, so they'd probably have the same surname if it applied to Dogs in the Land of Ooo.
After he is freed, he befriends and tells him where to find food and adventure.
Both Frankie and Mac did this often, with Mac sounding shocked and indignant surprising, considering he invented Bloo and Frankie just sounding enraged.